Daikin Altherma 3 Review
The Daikin Altherma 3 doesn't shout about itself. It sits quietly at the premium end of the market, matches the Vaillant Arotherm Plus on headline COP, and does something most other heat pumps struggle with: it keeps performing when the temperature drops hard. If you're in a colder part of the UK — northern England, Scotland, anywhere that regularly sees sub-zero winters — this deserves serious attention.
The flash injection advantage
Most heat pumps see efficiency drop significantly as outdoor temperatures fall below zero. The physics makes this inevitable — there's less heat energy in cold air to extract. Daikin's flash injection technology partially addresses this by injecting refrigerant vapour into the mid-point of the compression cycle, maintaining higher capacity and efficiency at low ambient temperatures.
In practical terms: at -7°C, a standard heat pump might be working hard to keep up. The Altherma 3 handles it more comfortably. For most of England and Wales this rarely matters — UK winters are milder than the test conditions suggest. For Scotland, northern England, or elevated locations, it's a genuine advantage.
The specs
| Spec | Detail |
|---|---|
| Refrigerant | R32 |
| COP (A7/W35) | Up to 5.1 |
| Capacities | 4–16kW |
| Installed price | £9,000–£13,000 |
| Warranty | 5 years |
A COP of 5.1 at A7/W35 matches the Vaillant Arotherm Plus — the highest on this list alongside it. The capacity range of 4–16kW is also one of the widest available, which is useful for both smaller well-insulated homes and larger properties with higher heat demand.
The outdoor unit is notably compact for its output range. If space is tight — a narrow side passage, a small courtyard, a busy utility area — the Altherma 3's footprint works in its favour.
Cold weather performance in numbers
Daikin publishes performance data at ambient temperatures down to -25°C. At A-7/W35 (outdoor temperature -7°C, flow temperature 35°C), the Altherma 3 maintains a COP comfortably above 3. Some competitors fall to 2.5 or below at that temperature. When you're running the system hardest — cold days, highest heat demand — the efficiency advantage is most meaningful.
The honest downsides
The 5-year warranty is shorter than Vaillant's 7-year standard (10 with registration). For a premium-priced unit, that's a legitimate criticism. Daikin should do better here.
The Altherma 3 is also less familiar to UK consumers than the Mitsubishi Ecodan. It's well-known among installers, but if you're doing your own research and asking around, you'll find less peer-to-peer feedback than you will for Mitsubishi. That's changing as the installer base grows, but it's worth knowing.
R32 refrigerant — same caveat as the Ecodan. It's serviceable and widely available now, but it's not where the industry is heading long term.
Who should buy this
- Anyone in Scotland, northern England, or exposed locations where cold-weather performance matters.
- Homes needing a wide capacity range — the 4–16kW span covers a lot of ground.
- Space-constrained installations where the compact outdoor unit is a practical benefit.
- Anyone who wants Vaillant-level efficiency without the Vaillant price — the performance numbers are comparable, the price slightly lower.
Who should look elsewhere
- If long warranty coverage matters to you, the Vaillant's 10-year option is hard to ignore.
- If installer familiarity in your area is a priority, Mitsubishi's larger UK network is an advantage.
- If you want natural refrigerant for future-proofing, you need the Vaillant Arotherm Plus.
The Altherma 3 is a quiet achiever in the best sense. If you're choosing on pure performance and cold-weather capability, it belongs at the top of your shortlist alongside the Arotherm Plus. For the full brand-by-brand comparison, see our best heat pumps UK guide. For costs after the £7,500 BUS grant, see our heat pump costs guide.